Field of the Technology
The invention relates to the field of medical devices and methods, namely an optical instrument and a computational model for scanning and imaging of human body composition including tissue water, lipid, oxygenated hemoglobin and deoxygenated hemoglobin content.
Description of the Prior Art
Diffuse optical spectroscopic imaging (DOSI) methods provide a low-cost, non-invasive approach for obtaining critical information regarding the structure and function of tissue. They use Near Infrared (NIR) light between 650 and 1000 nm to interrogate tissue to depths of several centimeters beneath the surface including the brain, breast, bone, abdomen and muscle. Also, the low energy output and non-ionizing radiation of the NIR spectrum causes no damage to the tissue, making DOSI a viable method for medical imaging purposes. Access to important physiological processes in the human body requires penetrating through 2-3 cm of tissue. DOSI provides information about tissue function and structure through the detection of four major components found in tissue: oxygenated hemoglobin, deoxygenated hemoglobin, water, and lipids
Tissue acts as a highly scattering turbid medium with low absorption when interacting with NIR light. As photons enter the tissue, they undergo multiple scattering and absorption events that cause the photons to diffuse in random directions. Diffusion models have been developed for light-tissue interactions to study subsurface tissue characteristics. Three main modalities currently exist for measuring tissue optical properties: continuous wave (CW), frequency domain (FD), and time domain (TD) imaging. The CW (time-unresolved) method provides qualitative information by measuring only relative changes in tissue components. This technique provides fast measurements and simple circuit designs, but is unable to separate scattering from absorption in a single measurement. Moreover, these techniques assume constant scattering and neglect possible changes in scattering occurring during a continuous measurement. This assumption can introduce significant errors when accurately calculating absorber concentrations in the tissue.
In contrast, TD and FD methods (time-resolved) provide quantitative approaches to optical imaging by separating absorption from scattering. A TD technology implements a short pulse beam (<100 ps) into tissue that broadens as it reaches the detector due to the scattering and absorbing events within the tissue. Despite its ability to obtain both scattering and absorption information, time domain imaging has a few limitations that prevent the translation of this technology to a portable real-time clinical friendly system. TD's optoelectronic high cost and complex circuitry reduces spectral bandwidth; thereby in applications such as breast cancer, information about water and fat content are inaccessible.
The FD modality implements the Fourier transform of the TD approach. On the source side, the FD system modulates the light source intensity with a Radio Frequency (RF) signal as the light enters the tissue. On the detector side, the AC amplitude, DC average intensity, and phase shift are measured using photon detectors. These amplitude and phase measurements are made at multiple frequencies and are subsequently fed into a frequency-domain diffusive analytical model of light propagation for a (semi)infinite medium to extract optical properties (absorption and scattering). FD also has limited spectral bandwidth similar to TD modality. However, FD circuit complexity, cost, and size are improved in comparison to TD. Because a limited number of wavelengths can affect the recovering of chromophore concentrations significantly, a large wavelength range is required. Achieving this goal, covering a large spectral bandwidth, using time-resolved techniques requires tunable sources or a large collection of laser diodes resulting in a bulky slow expensive system with complex maintenance requires tunable sources or a large collection of laser diodes resulting in a bulky slow expensive system with complex maintenance.
One strategy for overcoming both time-resolved and time-unresolved technique limitations is development of a hybrid method that utilizes both modalities in tandem to extract near-infrared absorber concentrations accurately. Our group has developed a combined broadband quantitative platform to recover absolute NIR absorption and scattering spectra of biological tissues. The quantitative information is provided by the Frequency Domain Photon Migration (FDPM) module while large spectral bandwidth from 650 nm to 998 nm with step of 0.5 nm (697 wavelengths in total) is provided by the steady state module. Four tissue chromophore concentrations are extracted from broadband spectra. Although this platform is powerful and has rich information content, however it has a few limitations such as speed, cost and size. Depending on media attenuation and required source-detector spacing, a single tissue measurement with this system can take up to 5 seconds.
What is needed is an apparatus and method for expanding spectral bandwidth and improving acquisition speed in diffuse optical spectroscopic imaging which also improves system costs and dimensions in order to lower barriers to clinical access. The apparatus and method should also be an inexpensive integrated method for continuous spectroscopic imaging in human tissues.